1. Field of the Invention
This invention relates to a wood stove which burns wood efficiently to combust hydrocarbons and gas by-products substantially and completely. More particularly, the invention concerns a wood burning stove which produces low levels of particulate emissions without a catalytic converter.
2. General Discussion of the Background
Wood stoves have become an increasingly popular means for heating homes and other structures. This popularity has created environmental problems because the wood burned in the stoves is incompletely combusted. The result is that particulate emissions such as unburned hydrocarbons, carbon monoxide and other gases, are expelled as smoke through the flue of the wood stove into the environment. This type of pollution creates smog and presents serious environmental problems in those parts of the country where wood burning stoves are extensively used.
Many regulatory agencies have begun to address this problem by limiting the acceptable amounts of particulate emissions from wood stoves being sold. The Oregon Department of Environmental Quality, for example, has limited particulate emissions for conventional wood burning stoves to 15 grams per hour after June 30, 1986, and only 9 grams per hour after June 30, 1988.
The wood stove industry has reacted by introducing catalytic wood burning stoves which cause smoke to ignite and burn at much lower temperatures than usual within the wood stove. Incompletely combusted material in the smoke is thereby more completely consumed, and more heat is produced by the same fuel load. Examples of such catalytic systems are U.S. Pat. No. 4,438,756 and U.S. Pat. No. 4,520,791. Such catalytic systems, however, have several drawbacks. The catalytic combustor is expensive, and therefore increases the original cost of the stove. The catalytic combustor is also exhausted after several years of use and must be replaced at a substantial additional cost. These factors have limited public acceptance of catalytic combustors in spite of their greatly reduced levels of particulate emissions. There is accordingly a need for a noncatalytic wood burning stove having low levels of particulate emissions that satisfy environmental regulations.
The Bosca FS 500 wood stove manufactured by Brugger Industries Ltd. of Wellington, New Zealand, has attempted to meet this need. The FS 500 is a wood burning stove having primary, secondary, and tertiary air supplies that feed oxygen to the fire at different positions along a flow path within the firebox. The primary air supply is fed to the burning pile of wood where primary combustion takes place, and smoke (which contains uncombusted materials) is generated. As the smoke moves upwardly in the firebox, it passes an elongated secondary air inlet that feeds additional air to the smoke and promotes secondary combustion. A tertiary supply of air is introduced into the flow path of the smoke downstream of the secondary supply to further promote burning and reduce particulate emissions to about 13.8 grams per hour.
Another approach to the problem involves regulating the stove damper so that the damper always remains substantially open. The temperature at which a fire burns is inversely proportional to the amount of particulate emissions produced. A very hot fire completely burns wood and smoke, while a cooler fire produces smoke with insufficient heat to ignite the particulates and gases in the smoke. It is therefore possible to reduce particulate emissions by governing the damper on a wood stove such that the fire will burn only at a vigorous, high heat producing level. This approach has the obvious drawback of limiting the range of heat output of the stove and reducing its effectiveness in comfortably controlling the temperature of a dwelling.
Neither the Bosca FS 500 nor stoves with dampers governed as aforesaid can achieve less than about 13.5 grams per hour particulate emissions without a catalytic combustor. It is accordingly an object of this invention to provide such a noncatalytic stove that efficiently combusts solid fuel and smoke particulates to reduce pollutant emissions to less than about 13.5 grams per hour.
Another important object of this invention is to provide such a wood burning stove that reduces particulate emissions without relying on exhaustible materials such as catalytic combustors that must be periodically replaced at great expense.
Still another important object of this invention is to provide such a stove that can reduce particulate emissions while continuing to operate over a broad range of heat outputs with the damper at varying positions.